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Calculation formula for usable disk storage

Is there a formula I can use in Excel to determine the usable space that I'll achieve with various hard drive sizes, raid configs, stripe size, filesystem type, and allocation/cluster sizes and virtual files.

For example, trying to figure out how much usable space I'll have without having to buy the system of the following:

8 x 600GB SAS drives
RAID10
Stripe Size 64K (RAID)
NTFS filesystem
Windows allocation/cluster size: 64K
Then creating a 200GB hyper-v disk file on that volume.

And then figure out the same for 2 x 2TB drives, or with different RAID types/configs.

Thanks!
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David
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It depends on how accurate you need to be.   It will be impossible (virtually) to obtain exact numbers because you are missing other variables, some are controller/vendor/product specific.  Then you have to deal with "hidden" / reserved areas that are functions of the number and sizes you put on a specific file system based on additional filesystem parameters and the number and degree of fragmentation of files.

With 8x600 in R10 config you get RAW usable of 4x600GB.  With a 2x2000 RAID1 you get raw usable of 1x2000Gb.


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MrVault

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thanks. yeah, I knew the raw numbers (since RAID10 and RAID1 use give half the space). A department here needs 2150GB free space on a volume in RAID10. I don't know if I buy 8 600 GB drives if that will give enough.
yes, it will be enough. You left a sufficiently large fudge factor
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besides experience and going based on gut, how does one come to this conclusion? I think you're right, but what if the department said they needed 2350 free space, or 2300? see what I mean?
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Duncan Meyers
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Unfortunately getting this department to really explain why they need XYZ usable space is like pulling teeth.

Thanks for the info. I realized most of this (that 100GB is not going to be really 1,000,000K
Well you could always try a different technique to give you a wide, comfortable fudge factor.  As them to define "usable".  Do they mean that if you give them a system with exactly 2,350,000,000 bytes, that they can read and write all day long, that they are willing to ignore that the O/S will probably crash and whatever applications they run will blow up if all 2.3TB was actually written?  


Ask them how much free space every program they plan on using requires, in worst case scenario, down to the byte to insure that programs don't die or slow down to the point they are unusable when the 2,350,000,000.   Then ask anybody foolish enough to even try to respond, how many users does that represent, and does it change based on the size of the amount of data they have. Ask if you can have the equation based on the number of data files each application has, so you can give them the precision they desire using a spreadsheet.  Do they want to print files? Ask for the list of files and say they all have different amounts of overhead.

 Somebody will take the bait and tell you to estimate, then you can fire back with  buying in multiples of 500GB and saying it is a decent estimate.

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Thanks. I can only imagine the looks I'd get if I went that route with asking them to estimate. However I can also vividly see the guy who's head will explode if in fact he gets in there and see less than 2400GB free. I'm going to bank on letting his head explode instead. I'll just claim it was impossible to get more without degrading performance significantly (RAID 10 to RAID 5).

as for the file size, that's good point. Although in our case this volume will hold hyperv VM files, so the file size will definitely be greater than 64K. I always wondered what I should format the drive in the VM as that's inside of the file.

So on host, V Drive is 64K formatted with VM files on it. Each VM file represents a disk presented to a volume (either boot or data disks). When I create a 200GB VM disk as the E drive for the VM and boot up the VM, do I format that E drive as 4K or 64K? The E drive will host SQL databases. I realize virutalized SQL cannot perform as well as non-virtualized, but I still need to get the best performance I can. Typical SQL recommends 64K allocation size for SQL databases and log files, but since inside a single VM disk file, does that even matter anymore? I realize we're getting a bit off topic - but it's fun! :)
64KB for sure.
For SQL, 64K for both the host and VM formatting. Hopefully you ar using Hyper-V 2008 R2 or R2 SP1, in which case the VHD can by dynamic. Keep in mind that is you get close to filling up the host volume that it will take the VM offline, so don't put a 200 GB VHD on a 200 GB volume and expect that everything will be okay. I would make sure that at least 10 GB is free onthe parent volume.
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if I make the vhd dynamic, will it impact performance as it grows? if I set it to fixed at 200GB and MBT size, can I expand it later with diskpart if needed?
it won't make much of a difference, just give you slightly more random I/O, less opportunities for read caching, but you probably won't have a lot of that anyway, so go for it.
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do you know if I can I expand a fixed disk that's formatted as mbt?
yes or no. it depends on where the new space comes from.  it starts with the raid, you deal with the 2tb limit also.   lots of rules, but possible in many situations but not all
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so besides not wasting space up front is there any other benefit to doing dynamic? if it can be expanded later, I'm inclined to give it 200GB up front so that I don't think there's more space than we can afford long term.
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Not sure if we got confused somewhere. I'm not using ESX. I'm using HyperV on Windows Server 2008 R2. also, the total space for the host volume needs to be be 2TB+. The VM disks don't need to be that large. Only 200GB to start.
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thanks kevin. just the clarification I was looking for. we have plenty of free space for now, but don't want them to see that free space and didn't want to over subscribe so we did it fixed. thanks everyone!
Thanks! Glad I could help.